(1) Field of the Invention
This invention relates to a fuel cell system, especially to a fuel cell system which is started smoothly.
(2) Description of the Prior Art
In a conventional fuel cell system, a hydrogen gas as a fuel is supplied from a high-pressure hydrogen gas cylinder to a fuel cell through a hydrogen gas supply path equipped with a pressure reducing valve for reducing a pressure of the hydrogen gas to a specified level.
The above fuel cell system has the problem of low operability, which is caused by the following reasons: (a) the hydrogen gas cylinder, which can accommodate only a small amount of hydrogen gas in a unit volume thereof, has to be replaced frequently; and (b) the cylinders are hard to handle.
It has been proposed that alcohol or naphtha, which is used as the fuel, is supplied through a fuel supply path equipped with a fuel reforming apparatus for turning the alcohol or naphtha into a hydrogen gas. Such a method increases a volume density of the fuel but requires a huge and expensive system due to the large-scale fuel reforming apparatus.
Also it has been proposed to supplying a hydrogen gas from a tank accommodating a hydrogen absorbing alloy, which is provided instead of the high-pressure hydrogen gas cylinder. Since the hydrogen absorbing alloy absorbs heat when desorbing the hydrogen gas, a hydrogen gas absorbing/desorbing equilibrium pressure (will be referred to simply as equilibrium pressure) is lowered, thereby causing an amount of desorbed hydrogen gas to fluctuate unless heat is supplied from the outside.
Japanese Patent Publication No. 56-26113 and U.S. Pat. No. 4,826,741 have disclosed supplying an exhaust heat from the fuel cell to the hydrogen absorbing alloy in order to prevent the decline of the equilibrium pressure. This system has no problems when the system is in a steady-state operation. When the temperature of the exhaust gas is low, such as during the operation starting time, the supplied heat is not enough to raise the equilibrium pressure, thereby preventing a smooth supply of the hydrogen gas.
The following two systems have been proposed in order to solve the above problems.
(A) Using a hydrogen absorbing alloy having a high equilibrium pressure.
(B) Providing an inner container and an outer container as disclosed in Japanese Patent Publication No. 62-49703. A hydrogen absorbing alloy is accommodated in one of the containers and a metal hydride is accommodated in the other. A hydrogen gas is supplied to one of the containers, whereby the container absorbs the hydrogen gas. Heat generated by this absorption is used to dissociate a hydrogen gas in the other container. The dissociated hydrogen gas is supplied to the fuel cell.
The above two systems also have problems.
(Problem of A) As an ambient cell system is increased, a temperature of the hydrogen absorbing alloy is also increased, thereby drastically raising the pressure of the hydrogen gas. Since this phenomenon requires a high pressure tank for accommodating the hydrogen absorbing alloy, the manufacturing cost of the system is high.
(Problem of B) The supply of the hydrogen gas to one of the containers requires an external source of the hydrogen gas. Such a construction is not desirable for a compact, portable system.